Many examples of the apparatus in related art are embodiments of the Carlson dry contact method in which a powder developing agent is used as the developing agent that develops visible images from electromagnetic latent images. The apparatus transfers and fixes the toner used to make visible images from the electrostatic latent images, formed by electrophotographic or other methods, to the paper or other recording medium. This method is used because of advantages such as the high energy amplification factor and high processing speeds. Disadvantages of this method include the amount of toner dispersion caused by use of a powder developing agent, and the poor resolution caused by the large size of the toner particles (7 to 10 .mu.m). In addition, the powder does not flow easily and is difficult to stir, which makes it difficult to obtain uniform images over a wide range.
Accordingly, there is a need for wet developing when higher resolutions and improved tonal reproduction are required. The size of the toner particles used in wet developing is 0.1 to 0.5 .mu.m, which is about a tenth of the size of the particles in dry developing agents. In addition, the toner has a high charge amount and toner image inaccuracies do not easily occur.
However, the liquid developing agent usually used in the wet style of electrostatic recording apparatus has low viscosity. These low-viscosity liquid developing agents consist of ISOPAR G.RTM. (ISOPAR G.RTM. is a registered trademark of the Exxon Corporation), an organic solvent, in which toner is mixed at a proportion of about 1 to 2%. Since the proportion of toner is small, the above style of apparatus in related art requires large quantities of the liquid developing agent, which makes reductions in the size of the apparatus difficult. In addition, the ISOPAR.RTM. (registered trademark) used as the dielectric liquid (carrier liquid) is highly volatile and releases an unpleasant odour. Therefore, use of the apparatus of related art adversely affects the work environment and has negative environmental effects.
Hence, it is desirable to develop a developing method that uses a non-toxic and odourless carrier, such that the apparatus does not need to be tightly sealed, and that uses a more highly concentrated liquid developing agent. However, many aspects of the technology concerning use of a highly concentrated liquid developing agent with electrostatic latent image development methods were heretofore unknown. For example, development of techniques to improve the selectivity between image parts and non-image parts for toner adhesion during the development process and to improve performance during the separation process was required.
In the wet methods of developing electrostatic latent images in related art, the electrostatic latent images formed on the image bearing member are made into visible images by toner, which consists of charged developing particles. In these methods, use of the same distribution medium as the medium used to distribute the liquid developing agent to apply a pre-wet liquid on the image bearing member before the actual developing process begins is a well known means of preventing the adhesion of toner to the non-image parts of the image bearing member and thereby preventing toner inaccuracies. Various methods can be used to apply the pre-wet liquid. For example, the same method as that used to supply liquid developing agent to the latent image surface of the image bearing member, in which the developing agent bearing member is a sponge roller, may be considered. Alternately, a roller with depressions and protuberances may be used as the member that supplies the pre-wet liquid. This roller is brought into contact with the image bearing member in order to apply the pre-wet liquid (Japanese Unexamined Patent Publication No. 60-147751 of 1985). Or, a blade provided with a slit from which pre-wet liquid flows may be used. In this method of applying the pre-wet liquid, the blade is positioned near to but not touching the image bearing member such that the pre-wet liquid forms a liquid bank between the image bearing member and the blade (Japanese Unexamined Patent Publication No. 4-1687 of 1992).
However, in order to achieve an extremely thin and uniform film of pre-wet liquid, the roller and image bearing member touch in the method disclosed in Japanese Unexamined Patent Publication No. 60-147751 of 1985. This damages the image bearing member and lowers developing performance. In the method disclosed in Japanese Unexamined Patent Publication No. 4-1687 of 1992, it is difficult to maintain a fixed distance between the blade and the image bearing member. Therefore, the pre-wet liquid cannot be applied at a uniform thickness and, consequently, it is impossible to adequately prevent toner adhesion to the non-image parts on the image bearing member.
The above types of pre-wet application methods in related art can be used when the liquid developing agent has low viscosity and consists of the organic solvent ISOPAR G.RTM. (registered trademark) in which toner is mixed at proportions from 1 to 2%. However, the types of methods that satisfactorily prevent toner from adhering to the non-image parts on the image bearing member when a liquid developing agent such as the one implemented in this invention is used, that is, when the liquid developing agent has the toner dispersed in the dielectric liquid at higher concentrations and has a higher viscosity of between 100 and 10,000 mPa.s, were heretofore unknown. Since the adhesion of this type of highly viscous liquid developing agent to the surface of the image bearing member is greater, development of an appropriate method for preventing toner adhesion was required.
In related art, a charged transfer member that carries an electric charge of opposite polarity to the toner is brought up against the image bearing member through the medium of the recording medium in order to transfer the toner image, which has been formed on the surface of the latent image on the image bearing member, to the recording medium.
When liquid developing agents are used, the image that is transferred to the recording medium can spread if the pressure of the recording medium against the image bearing member is too high. The measures heretofore taken to deal with this problem relate to the liquid developing agents usually used in the wet style of electrostatic recording apparatus and other apparatus in related art, that is, to liquid developing agents that have low viscosity and that consist of the organic solvent ISOPAR G.RTM. (registered trademark) in which toner is mixed at proportions from 1 to 2%. However, when highly concentrated liquid developing agents with high viscosity are used, the types of methods that satisfactorily transfer the toner image formed on the surface of the latent image on the image bearing member to the recording medium without the image spreading were heretofore unknown.
This invention is a response to the above mentioned circumstances and is intended to provide a liquid developing method and liquid developing apparatus that will reduce pollution, improve the work environment, and enable easy development of electrostatic latent images at high resolutions in apparatus of reduced size.
Another aim of this invention is to provide a liquid developing method and liquid developing apparatus for developing electrostatic latent images such that pre-wet liquid can be applied at a uniform thickness to the image bearing member without causing damage to the image bearing member and such that, as a result, toner can be prevented from adhering to the non-image parts on the image bearing member.
A further aim of this invention is to provide a liquid developing method and liquid developing apparatus for developing electrostatic latent images such that, when a highly viscous liquid developing agent in which toner is dispersed at high concentrations is used and the liquid developing agent layer formed on the developing agent bearing member touches the pre-wet liquid layer formed on the image bearing member during the developing process, the pre-wet liquid layer and the liquid developing agent layer form a two-layer structure in order to prevent the occurrence of inaccuracies in the liquid developing agent layer and to allow full utilization of the function of the pre-wet layer such that, as a result, the adhesion of toner to the non-image parts of the image bearing member and the occurrence of inaccuracies in the image can be prevented.
Yet another aim of this invention is to provide a liquid developing method and liquid developing apparatus that can prevent toner image inaccuracies occurring when the toner image formed on the latent image surface of the image bearing member touches the recording medium and that can, as a result, transfer the toner image to the recording medium without the image spreading.